Problem: The side length of square $A$ is 36 cm. The side length of square $B$ is 42 cm. What is the ratio of the area of square $A$ to the area of square $B$? Express your answer as a common fraction.
Answer: The ratio of their areas is going to be the ratio of their side lengths, but squared. The ratio of the side length of square A to that of B is $\frac{36}{42}=\frac{6}{7}$. Thus, the ratio of their areas is $\left( \frac{6}{7} \right) ^2=\boxed{\frac{36}{49}}$.